Nanoscale semiconductor lasers are poised to become the driving force toward achieving dense integration of photonic components on a semiconductor chip for future on-chip photonic sensing and communication capabilities. Miniaturization of photonic components is likely to have a huge commercial impact and will present many future directions for the field of optoelectronics.
A plasmonic laser is a type of laser that confines light at a sub-wavelength scale by storing some of the light energy through electron oscillations referred to as surface plasmon polaritons (“SPPs”). Plasmonic lasers are lasers that lead to stimulated emission of SPPs. Plasmonic lasers are sometimes referred to as SPASERs (“Surface Plasmon Amplification by Stimulated Emission of Radiation”). Quantum cascade lasers (“QCLs”) are nanostructured semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum. (QCLs relate to the gain medium used.) Unlike typical interband semiconductor lasers that emit electromagnetic radiation through the recombination of electron-hole pairs across the material band gap, QCLs are unipolar and laser emission is achieved through the use of intersubband transitions in a repeated stack of semiconductor multiple quantum well heterostructures.
Metal-cavity (also referred to as metal-clad or metallic) semiconductor lasers are a type of plasmonic laser that incorporate metallic cladding surrounding the semiconductor gain medium and are the most promising types of nanoscale lasers at near-infrared wavelengths today. Such lasers will be useful for achieving dense integration of photonic components on a semiconductor chip for future on-chip photonic sensing and communication capabilities.
Metallic cavities are also utilized for the best performing semiconductor lasers at far-infrared wavelengths (also known as terahertz or THz frequencies). In this wavelength range, terahertz/THz quantum cascade lasers (QCLs) are by far the best performing lasers compared with other types of compact sources of far-infrared (or THz) radiation. These types of lasers are relatively new, but recent progress in their development have brought them tantalizingly close to commercial applications in the area of terahertz sensing, imaging, and spectroscopy. THz metal-clad QCLs are plasmonic lasers that have potential applications in biological and medical sciences, security screening, illicit material detection, non-destructive evaluation, astrophysics and atmospheric science, manufacturing and process control, communications and ultrafast spectroscopy.
Metal-clad semiconductor lasers operate by confining light at sub-wavelength dimensions thereby storing some of the light energy through electron oscillations called surface plasmon polaritons (“SPPs”). At least one metal surface is required in order to generate and sustain SPPs in the laser's cavity.
Metallic cavities supporting SPP modes have been used to realize nanoscale plasmonic lasers (or “spasers”) for potential applications in integrated optics. Such cavities are also used for THz QCLs to achieve low-threshold and high-temperature performance. The most common type of plasmonic lasers with long-range SPPs, which include THz QCLs, utilize Fabry-Pérot type cavities in which at least one dimension is longer than the wavelength inside the dielectric medium of the cavity. A common challenge for all such plasmonic lasers (regardless of the wavelength of their operation) is the poor coupling of SPP modes in the cavity to free-space radiation, which results in highly divergent radiation and poor radiative efficiency from the plasmonic laser. Cavities with periodic photonic structures and broad-area emission are therefore used for narrow-beam emission. However, Fabry-Pérot cavity structures with sub-wavelength apertures are more desirable, especially for electrically pumped plasmonic lasers, which have highly divergent beams.
Distributed feedback (“DFB”) is a technique often applied to semiconductor lasers in order to obtain optical emission at a single frequency (i.e., in a single-spectral mode). DFB lasers use one- or multi-dimensional interference gratings in order to provide optical feedback for the laser by the mechanism of Bragg scattering.
FIG. 1A, which is prior art, shows an example of a periodic grating implemented in the top metal cladding of a parallel-plate metallic cavity that could be utilized to implement p-th order DFB (where p is an integer) in the cavity by choosing the appropriate periodicity. Metal-clad DFB laser 100 comprises cladding 102(1) and cladding 102(2) and apertures 106(1)-106(5) in cladding 102(1). Cladding 102(1) and 102(2) is typically composed of a metallic material. Metal-clad DFB laser 100 is embedded in surrounding medium 116.
FIG. 1B, which is prior art, illustrates phase-mismatch between successive grating apertures for SPP waves on either side of the metal cladding of a metal-clad cavity that could be utilized to implement p-th order DFB by choosing the appropriate periodicity. As shown in FIG. 1B, the guided SPP wave 108(1) inside the optical cavity 104 diffracts out through apertures 106(1)-106(5) and generates single-sided SPP waves 108(2) that are supported by the cladding 102(1) in surrounding medium 116. Phase mismatch (phase change of
  (            ⅇ              ⅈ        ⁢                                  ⁢        p        ⁢                                  ⁢        π              ⁢                  ⁢          vs      .                          ⁢              ⅇ                  ⅈ          ⁢                                          ⁢          p          ⁢                                          ⁢          π          ⁢                                          ⁢                                    n              s                                      n              a                                          for propagation length of one grating period) is exhibited between SPP waves on either side of metal cladding in which the grating is implemented, i.e., 108(1) and 108(2) respectively. Here, ns is the propagation index of the guided SPP wave in the surrounding medium (which is approximately equal to the refractive index of the surrounding medium), and na is the propagation index of the guided SPP wave inside the metallic cavity (which is approximately equal to the refractive index of the dielectric medium, also referred to as the active or the gain medium, inside the cavity).
FIG. 1C, which is prior art, shows a portion of a metal-clad plasmonic laser incorporating DFB. As a result of the phase mismatch illustrated in FIG. 1B, a coherent single-sided SPP wave 108(2) in surrounding medium 116 cannot be sustained due to destructive interference with guided SPP wave 108(1) inside the optical cavity 104.
Applicant has identified significant shortcomings with conventional approaches to plasmonic lasers. Because plasmonic lasers emit radiation in multiple directions (i.e. highly divergent radiation), they exhibit very poor beam quality. In particular metal cavity semiconductor lasers suffer from poor beam quality (beam spread) due to sub-wavelength dimensions of the radiating apertures in the metallic cavity. This arises due to divergence effects generated by diffraction. In general, single-mode, low divergence and high power emission is difficult to realize when a laser cavity is of sub-wavelength dimensions. This is true for many types of nanocavity and microcavity lasers such as THz QCLs and limits their practicability. Current THz semiconductor lasers suffer from poor beam quality and highly divergent beams (divergence angle of more than 90 degrees) due to the sub-wavelength dimensions of the apertures in Fabry-Perot type metal cavities. There exist several approaches of implementing DFB, which can improve the beam quality of single-mode THz semiconductor lasers including second-order and third-order DFB methods and two-dimensional photonic-crystal structures. Other techniques may also be applied such as plasmonic collimation with periodic photonic structures that are implemented outside of the lasing cavity. However, all of these conventional approaches exhibit various disadvantages.
Plasmonic lasers with second-order DFB can only achieve a narrow beam pattern in one direction whereas the beam is divergent in the other direction (see, for example, Fan et al. Optics Express v. 14, p. 11672 (2006), Kumar et al. Optics Express v. 15, p. 113 (2007), and Sirtori et al. Nature Photonics v. 7, p. 691 (2013)). Such beams are not suitable for most applications. This is true for plasmonic lasers with practical cavity dimensions in which the laser cavity is kept narrow to keep the pump power small and to maintain good heat-extraction from the laser, such that continuous-wave (“cw”) operation could be achieved.
Plasmonic lasers with third-order DFB (Amanti et al. Nature Photonics v. 3, p. 586 (2009)) can achieve narrow-beam emission when the propagation index of the SPP wave inside the cavity na≈3. For THz QCLs this so-called phase-matching condition can be satisfied by a complex deep etching technique to lower the original refractive index of the active region to achieve a narrow beam output (Kao et al. Optics Letters v. 37, p. 2070 (2012), Hu et al. U.S. Pat. No. 9,036,674 B2 (2015)). Furthermore, there exist fabrication challenges with a third-order DFB scheme in achieving a desired phase-matching condition, which may not be precisely satisfied due to the dependence of the phase-matching condition on the lasing frequency. Another disadvantage of the third-order DFB scheme as opposed to the antenna feedback scheme is that output power is lower in the former due to inefficient radiative outcoupling.
With plasmonic or meta-material collimators (Yu et al. Nature Materials v. 9, p. 730 (2010), Liang et al. Scientific Reports v. 4, p. 7083 (2014)), a large area of the semiconductor substrate is utilized for collimation of radiated power from a single laser ridge and hence this scheme is not appropriate for multiple lasers on a single chip. Also, this scheme has only been demonstrated for multi-mode lasers, and will lead to additional challenges when implemented for single-mode plasmonic lasers.
Plasmonic lasers with multi-dimensional photonic crystal structures suffer from requiring a large two-dimensional surface area for emission to achieve a narrow beam output (Chassagneux et al. Nature v. 457, p. 174 (2009), Halioua et al. Optics Letters v. 39, p. 3962 (2014)). A large surface-area laser suffers from temperature degradation in continuous-wave (“cw”) operation of the laser due to inefficient heat removal from the laser cavity during operation.